GB1320058A - Data transmission system - Google Patents

Abstract

1320058 Telegraphy INTERNATIONAL BUSINESS MACHINES CORP 19 April 1971 [19 Feb 1970] 20335/71 Heading H4P A data transmission system comprises a group of terminals A...N connected serially to a central station 1 which is arranged to transmit to one end 7 of the series connection a tone signal of frequency or frequencies outside the date frequency range to initiate data transfer from the terminals, wherein each terminal comprises a bandstop filter 13 which is operative, when the terminal has data to transmit, to prevent transmission of the tone signal to the next terminal. Using MODEMS 2, 12, central station 1 can send data over line 7 to the terminals and receive data from the terminals over line 8. A terminal wishing to send data actuates a switch 10 to connect filter 13 into line 8. The tone on line 7, is passed by bandpass filter 6 (which would block data) and detected at 11 in the first terminal wishing to send data. After the terminal has sent its data it reverses switch 10 and the tone proceeds to the next terminal. When the tone is detected at 5 at the central station 1, the latter stops sending the tone (this permits any terminals now wishing to send data to actuate their switches 10 anew) and may send data to the terminals. The tone can be one or several frequencies. Tone detectors 11 may be connected to line 7 instead, with their corresponding switches 10 also in this line. Different tones may be used for different groups of terminals, permitting some terminals to be polled more frequently than others, each filter 13 blocking only the frequency band occupied by the tone relating to its terminal. Further terminals may be addressed in a conventional way.